Multilayer coatings made up, for example, of a primer, a surfacer, a base coat and a clear coat layer are typical coating structures in vehicle coating and vehicle repair coating. Adhesion of the whole multilayer coating to the substrate is very important. Therefore, primer solutions and undercoat systems are needed with excellent adhesion and corrosion resistance, which are suitable for direct coating of metal substrates.
Currently most of the repair coating processes down to the metal use polyvinylbutyral-based wash primers and two-component polyurethane surfacers in order to achieve acceptable adhesion and good quality of the applied films. A major disadvantage of such processes is insufficient productivity. Normally it takes at least 3 to 4 hours air drying until the surfacer can be sanded and further operations can be started. Sanding after shorter air drying times may lead to films that are still too soft and vulnerable, increasing the risk of damage to the freshly applied paint system. The curing process can be accelerated by using IR radiation or evaluated temperatures. But accelerating the process that way consumes energy and curing equipment needs to be available. Also, adhesion of subsequently applied waterborne basecoats onto two-component polyurethane surfacers with high pigment/extender load is often weak. In addition wash primers generally have a high content of organic solvents, which is not desired for environmental reasons.
A further option is to use fast drying polyurea surface compositions. However, when applying two-component polyurea surfacers directly to metal substrates, specifically to steel and aluminum substrates, adhesion is in general rather weak and adhesion failures are frequently observed. Solutions to improve adhesion of polyurea coating compositions on metals have been disclosed, for example, in WO 2010/112157. WO 2010/112157 discloses a coating composition with low pigment load which can be applied directly to metal surfaces. The coating composition contains polyaspartates and silane-functional polyisocyanates. However, high pigment and extender loads which are needed to ensure good sandability of sanding surfacers can deteriorate adhesion of those compositions to the metal substrate.
Furthermore, DE 10 2006 052 919 discloses a process for conversion pretreatment of metal surfaces, wherein the metallic surfaces are pretreated with a phosphating solution containing phosphate and zinc (II) ions and titanium and/or zirconium compounds. DE 103 22 446 discloses aqueous compositions for anticorrosion conversion pretreatment of metal surfaces. The compositions are based on phosphating solutions and contain in addition water-soluble titanium and zirconium compounds and a quantity of free flouride. The pretreated metals are used, e.g., in automotive construction and shipbuilding.
It has hitherto not been possible to provide productive surfacers with acceptable adhesion to metal substrates or a productive combination of an adhesion primer and a surfacer, which can be sanded after shortened air drying times and which do not simultaneously substantially impair other important coating properties.
Accordingly, it is desirable to provide a process for multilayer coating and multilayer repair coating of metal substrates that increases the productivity under air drying conditions and yields a coating structure with very good corrosion resistance and adhesion properties, e.g. satisfactory wet and dry adhesion to metal substrates as well as interlayer adhesion. It is desirable that Properties of the multilayer coating, such as sandability, drying performance and the productivity of the whole coating process are not impaired as a consequence. Furthermore, other desirable features and characteristics of the various embodiments will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.